1. Field of the Invention
The present invention relates to a brake condition detector for detecting a brake condition of a vehicle and a vehicle controller for performing vehicle control such as anti-skid control based upon a brake condition detected by the brake condition detector.
2. Description of Related Art
Conventionally, a vehicle brake system includes, for example, a master cylinder coupled to a brake pedal, a wheel cylinder provided to a brake mechanism of the vehicle and a reservoir for storing brake fluid, all of which are connected via fluid lines.
In this brake system, an anti-skid controller is provided with an inflow valve, for example, in a fluid line between the master cylinder and the wheel cylinder and an outflow valve in a fluid line between the wheel cylinder and the reservoir. These valves are switched and controlled to increase or reduce the pressure of brake oil in the wheel cylinder.
During the execution of anti-skid control by such a controller, brake force is intensified by gradually increasing pressure of brake oil by a certain amount .DELTA.P as shown in FIG. 17A through the operation of a pump and a solenoid valve in an oil hydraulic circuit with the brake oil being supplied from the master cylinder and the reservoir to the wheel cylinder. Brake condition is determined after detecting an amount of reduction in the vehicle wheel speed in response to the increased pressure based on the wheel speed and its acceleration component which represents its derivative.
That is, as shown in FIG. 17B, when a slip rate of a vehicle wheel over a road surface in an area in which such slip rate is smaller than a point at which a gradient of a coefficient of friction .mu., which denotes the adhesion between the vehicle wheel and the road surface, greatly varies, i.e., the .mu. gradient turning point, the amount of reduction in the wheel speed in response to the increased pressure is small because of increased road surface brake force due to increase in the slip rate. However, when a slip rate is in an area in which it is larger than the above-described turning point, balance between the road surface brake force and brake force is upset even by a slightly intensified pressure and thus, wheel speed is reduced to a point in which the wheels tend to lock.
In other words, it is not until a wheel speed is reduced that a brake condition can be judged.
As a result, as shown in FIGS. 17C and 17D, for conventional anti-skid control, temporal characteristics of the wheel rotating speed and the hydraulic pressure (wheel pressure) would form an irregular wavy shape.
A system which detects a point where the gradient of a coefficient of friction .mu. greatly varies based on the decrease of the wheel speed poses the following problems.
First, when the hydraulic pressure is gradually intensified and wheel rotating speed is reduced, pressure may need to be reduced and thus, a fixed brake condition cannot be kept.
That is, there is a need to repeatedly execute the processes of increasing pressure, detecting vehicle speed drop and decreasing pressure to constantly cope with the ever-changing road surface condition, and so, the above system poses problems related to brake distance and passenger comfort.
One other problem is that when the vehicle moves to a road having a different .mu. gradient coefficient which represents the maximum .mu., for example, when road surface conditions changes and a coefficient of friction .mu. of the road surface increases rapidly, the time period in which pressure is gradually intensified until a wheel rotating speed is reduced will be prolonged and thus, delays in the determination of the above-described turning point will affect the brake distance.